1. Field of the Invention
The present invention relates to novel microorganisms having an ability to produce xylitol or D-xylulose, and a method for producing xylitol or D-xylulose by using a microorganism having an ability to produce xylitol or D-xylulose. D-Xylulose is useful as a material for the production of xylitol, and xylitol is useful as a sweetener in the field of food industry and the like.
2. Description of the Related Art
The demand of xylitol which is a naturally occurring sugar alcohol is expected to increase in future. Xylitol is a promising low-calorie sweetener because it has lower calories and exhibits comparable sweetness compared with sucrose. In addition, because of its anti-dental caries property, it is utilized as a dental caries preventive sweetener. Furthermore, because xylitol does not elevate glucose level in blood, it is utilized for fluid therapy in the treatment of diabetes. For these reasons, it is expected that the demand of xylitol will increase in future.
The current industrial production of xylitol mainly relies on hydrogenation of D-xylose as disclosed in U.S. Pat. No. 4,008,825. D-Xylose used as a raw material is obtained by hydrolysis of plant materials such as trees, straws, corn cobs, oat hulls and other xylan-rich materials.
However, such D-xylose produced by hydrolysis of plant materials suffers a drawback that it is rather expensive, and it is arisen from high production cost. For example, the low yield of the hydrolysis treatment of plant materials leads to low purity of the produced D-xylose. Therefore, the acid used for the hydrolysis and the dyes must be removed by ion exchange treatment after the hydrolysis treatment, and the resulting D-xylose must be further crystallized to remove other hemicellulosic saccharides. In order to obtain D-xylose suitable for foodstuffs, further purification would be required. Such ion exchange treatment and crystallization treatment invite the increase of production cost.
Therefore, several methods for producing xylitol have been developed, which utilize readily available raw materials and generate little waste. For example, there have been developed methods for producing xylitol utilizing other pentitols as a starting material. One of such readily available pentitols is D-arabitol, and D-arabitol can be produced by using yeast (Can. J. Microbiol., 31, 1985, 467-471; J. Gen. Microbiol., 139, 1993, 1047-54). As a method for producing xylitol by utilizing D-arabitol as a raw material, there can be mentioned the method reported in Applied Microbiology., 18, 1969, 1031-1035, which comprises producing D-arabitol from glucose by fermentation using Debaryomyces hansenii ATCC20121, then converting the D-arabitol into D-xylulose using Acetobacter suboxydans, and converting D-xylulose into xylitol by the action of Candida guilliermondii var. soya.
EP 403 392A and EP421 882A disclose methods comprising producing D-arabitol by fermentation using an osmosis-resistant yeast, then converting D-arabitol into D-xylulose using a bacterium belonging to the genus Acetobacter, the genus Gluconobacter, or the genus Klebsiella, forming a mixture of xylose and D-xylulose from the D-xylulose by the action of glucose (xylose) isomerase, and converting the obtained mixture of xylose and D-xylulose into xylitol by hydrogenation. There is also disclosed the production of xylitol comprising preliminarily concentrating xylose in the mixture of xylose and D-xylulose and converting the xylose into xylitol by hydrogenation.
However, those methods for the production of xylitol mentioned above utilize D-arabitol produced by fermentation as a starting material, and convert it by multiple process steps. Therefore, the processes are complicated, and less satisfactory ones in view of process economy compared with the methods based on extraction.
Accordingly, there has been desired a microorganism which has an ability to produce xylitol or D-xylulose through a single step by fermentation starting from glucose as used in the production of other saccharides a nd sugar alcohols. However, such a bacterium having an ability to produce xylitol or D-xylulose has not been reported so far.
On the other hand, breeding of xylitol fermenting bacteria has been attempted by using gene manipulation techniques. International Publication WO94/10325 discloses production of xylitol from glucose by fermentation by using a recombinant microorganism obtained by introducing an arabitol dehydrogenase gene derived from a bacterium belonging to the genus Klebsiella and a xylitol dehydrogenase gene derived from the genus Pichia into an arabitol fermenting microorganism (yeast belonging to the genus Candida, the gunus Torulopsis, or the genus Zygosaccharomyces). However, while production of 15 g/L of xylitol from 400 g/L of glucose has been reported for the aforementioned recombinant microorganism, it does not reach a practically useful accumulation level. Moreover, the aforementioned recombinant microorganism is introduced with a gene derived from a different species, and ther efore information about its safety cannot be considered sufficient.